The object of this invention is an acoustic device capable of selective and non-linear filtering of sound, and placeable in the external auditory canal of a user by means of an earplug, and especially for use in a military or industrial environment.
One of the problems to be solved for those in the military is how to communicate between themselves, to detect, and localize and to identify the sources of exterior noises, all the while protecting their hearing against sudden noises, especially the high level noise of weapons (up to 190 dB SPL.)
There already exist ear plugs that one introduces into the auditory canal which greatly reduce, in the same manner, all sounds, whether they be sudden noise at a high level, environmental noise, or speech sounds at low levels.
The document FR-A-2050740 describes a noise filter, sensitive to a higher degree to the energy of language and to a lesser degree to background noise, and consisting of a capsule adapted to be inserted in a auditory canal of a user, and a vibrating element capable of discriminating frequencies positioned in the interior of the capsule, the filter also containing a diaphragm coupled to the vibrating element in an effective manner.
However, this device has a complex design and an elevated cost. In addition, the attenuation curves obtained from this device are not ideal for use in the military environment because it reduces low level noises and high level noises in the same manner.
The object of the document EP-A-0440572 is a sound transmitting device capable of selective filtering, and designed to be placed in the external auditory canal. This device consists of a tip, produced by molding from an imprint taken from the wearer and completely filling the external auditory canal. An acoustic valve is inserted into the tip. The device contains at least one acoustic valve placed at the end of a tube, and a section considerably larger than that of the tube. This valve plays the role of a resonance cavity, according to HELMHOLTZ'S resonator principle, the filter therefore being a filter of the fourth order of which the attenuation slope is 30 decibels per octave.
However, this device is of relatively complex design and presents a problem of capillary resistance due to the propagation of acoustic waves during their passage from the valve in the tube of smaller diameter which extends into the residual cavity. The result is that this acoustic filter device reduces in appreciably the same manner and identical intensity low level noises (human voice, environmental noise) and high level noises (sudden noise such as weapon noise.)